This invention relates generally to a method and apparatus for stabilizing the output of a radiation detection instrument. A particular aspect of the present invention is its ability to automatically stabilize the output of the radiation detection instrument by responding to a predetermined energy level that is sensed by the instrument due to a known type of radiation interaction event that occurs in the radiation detector assembly of the instrument.
Many instrumentation processes utilize radiation interactions as a method of measurement. The reliability of these instruments depends to a great extent on the stability of the instrument in converting a radiation interaction event in the radiation detector assembly into an electric signal to be analyzed for a given purpose. It is typically desirable that the amplitude of each electric signal precisely represent the respective amount of energy deposited by each corresponding radiation interaction event that takes place in the detector. If this representation is stable, then the instrument will function well under a variety of conditions. If it is unstable, however, the measurement performed by the instrument will not be reliable.
One type of radiation interaction used to make measurements is gamma ray attenuation. When gamma rays pass through a medium they can interact by photoelectric absorption, Compton scattering, or pair production. The composition of the medium and the energy of the gammas determine which of these reactions are possible, and which are prevalent.
When a photoelectric absorption event occurs, all the energy of the gamma is imparted to an inner orbital electron of an atom in the absorbing medium. The gamma disappears and an electron is ejected from its orbit. The ejected electron creates excitation and ionization in the absorber. The photoelectric reaction is most predominant at low gamma-ray energies (E) and in high atomic number (Z) materials. The photoelectric coefficient, tau, is approximately proportional to E.sup.-3 and Z.sup.4.
When a gamma undergoes Compton scattering, it only imparts part of its energy to an outer orbital electron, the rest of its energy is carried off by a degraded gamma, which may undergo additional Compton scattering or photoelectric interactions. The ejected electron creates excitation and ionization in the absorber. Compton scattering is predominant at medium gamma-ray energies and is nearly independent of the atomic number of the absorbing medium, except at high Z values. The Compton coefficient, sigma, is proportional to approximately 1/E.
If a gamma ray contains an energy of greater than 1.02 mega-electron-volts (MeV), it can undergo pair production. This consists of a gamma ray passing in close proximity of the short range nuclear forces of an atom and its energy being converted to mass, according to the equation E=MC.sup.2. In this case, two particles, an electron and a positron, are created and the gamma disappears. The electron and positron cause excitation and ionization in the medium. The pair production coefficient, kappa, has a threshold at 1.02 MeV and is approximately proportional to the natural logarithm of E (ln E) and Z.sup.2.
An example of an instrument that utilizes gamma-ray detection is a radioactive densimeter. This instrument is used, for example, in a fluid line to measure the density of the substance flowing through the line. The radioactive densimeter is used extensively in the chemical and petroleum industries for this purpose. A typical radioactive densimeter used in oil well servicing to measure the density of cement slurries includes an encapsulated gamma ray emitting source mounted in a shield on one side of a pipe and a radiation detector assembly mounted on the opposite side of the pipe. The radioactive densimeter operates on the principle that the more dense the fluid is, the more the radiation from the source will be absorbed by the fluid whereby less radiation will reach the detector, which will result in a decreased radiation interaction rate in the detector.
A specific type of radioactive densimeter utilizes cesium-137 (Cs-137), with a gamma-ray energy of 662 kilo-electron-volts (keV), as a source; and it uses a thallium-doped sodium iodide (NaI) scintillator, with an effective atomic number of 32, coupled to a photomultiplier tube as a detector assembly. With this particular instrument, approximately 10% of the total counts registered by the unit are due to photoelectric absorption of the primary gamma ray in the scintillator, approximately 90% are due to Compton scattering events, and no pair production events occur because the gamma-ray energy is below 1.02 MeV. Cesium-137 decays by emitting a monoenergetic gamma ray, 662 keV. When a photoelectric event occurs in the scintillator, it always absorbs all of this energy and deposits an unique amount of energy, 662 keV, in the scintillator to be converted to ultraviolet light. The photomultiplier tube collects the ultraviolet light and creates an electrical pulse proportional to the intensity of the ultraviolet light.
FIG. 1 shows a typical gamma-ray energy spectrum obtained from a NaI detector exposed to Cs-137 radiation. Notice that the full energy photopeak of 662 keV (i.e., the collection of photoelectric events) has a Gaussian distribution. This distribution is due mainly to the statistical variations that occur in the number of electron-volts required to create each ion pair in the scintillator and the efficiency of converting the energy of an ion pair to ultraviolet light.
This instrument works well as long as the electric pulse output of the radiation detector assembly is a true function of the radiation energy deposited in the NaI detector for each individual radiation interaction event occurring in the detector. This is of critical importance because most gamma-ray detecting instruments, such as the aforementioned radioactive densimeter, process the electric signal by utilizing a set lower level discriminator that allows all electric pulses above the set level to pass and be counted and all electric pulses below the set level to be rejected. The pulses that are passed are counted and the resultant density determination is a function of this count rate. If the output pulse of the detector assembly is not a constant function of the energy deposited by the radiation event in the NaI detector, then the number of pulses per unit time that pass the lower level discriminator and are counted will not be truly representative of the characteristic being measured (e.g., density of the fluid in the pipe). The output of the radiation detector assembly can be adversely affected by temperature changes, photomultiplier tube supply voltage drift, photomultiplier tube-to-NaI detector coupling changes and other faults that affect the radiation detector assembly output pulse height, producing erroneous density readings. Therefore, a method is needed to ensure that the electric pulse output of the radiation detector assembly is a known function of the radiation energy deposited in the NaI detector for each individual radiation interaction event. This method typically provides an error-correcting stabilization function between the radiation detector assembly output and the lower level discriminator.
Gain stabilization (or, more precisely, spectrum stabilization via stabilizing amplifier gain adjustments) has been used successfully in laboratory instruments for a number of years and commercial units are available to perform that function. However, in instruments used outside the laboratory environment, such as the radioactive densimeter in field use, this stabilization has not been feasible because the operator would have to interact with the stabilizing function and provide input to the system. This interaction requires training, time, and extra hardware and is not practical in most field applications.
A common method used for spectrum stabilization in the laboratory includes analyzing the count rates that occur in two windows, of equal width, placed equal distance on opposite sides of the photopeak centroid, as shown in FIG. 2. The counts are collected by means of two identical single-channel analyzers (SCA): one single-channel analyzer is set on the lower window, and the other single-channel analyzer is set on the upper window. The count rates from the two single-channel analyzers are analyzed by a processing unit which controls a variable gain amplifier. If the photopeak centroid is located at an equal distance between the two windows, then the two windows will lie on oppositely symmetrical portions of the photopeak. This will result in the two single-channel analyzers registering the same count rates, within normal statistical variations, and the processing unit setting the variable gain amplifier to unity gain; hence no correction is made. However, if for any reason, such as temperature change, high voltage or electronic drift, changes in couplings, etc., the position of the photopeak were to change, the count rates in the two single-channel analyzers would become unbalanced. This would result in a correction being made, in the appropriate direction, in the variable gain amplifier to bring the photopeak back to the center position between the windows. For example, if the amplitude of the incoming pulses started to decrease, the lower window would register more counts than the upper window; the processing unit would then increase the gain of the variable amplifier until the count rates of the two single-channel analyzers were equal. If the amplitude increased, the opposite sequence would take place.
Although the foregoing stabilization technique is known, it does not automatically first find the relevant peak in the spectrum and therefore does not provide fully automatic stabilization. Thus, there is the need for a stabilization method and apparatus that can also automatically locate a relevant feature of an energy spectrum and then automatically adjust the located feature to produce stabilized spectra. This should be done automatically, without operator input, and without error. This requires an intelligent system that can locate the relevant feature in the energy spectrum, determine its present position, calculate the needed correction factor, adjust the gain, and then continuously check the energy spectrum to ascertain that the feature is in the correct position. This should be automatically initiated when power is applied, and continued indefinitely, without locking on the wrong feature or requiring any input from the operator.